Field of the Invention
The present invention relates to monolithic nonlinear frequency converters based on second order nonlinear materials used for three wave mixing processes in an optical cavity. In particular, the invention is a tunable monolithic cavity-based frequency converter pumped by a single-frequency laser where cavity resonance(s) are achieved by independently changing the temperatures of different sections of the crystal, including the periodically poled section and one or more adjacent, non-poled regions.
Description of Related Art
Frequency converters based on second order nonlinear materials can use a cavity consisting of mirrors aligned around a nonlinear material so that the beam(s) involved in three wave mixing processes are resonant. In monolithic nonlinear devices on the contrary the active material is not surrounded by separate mirrors that have to be aligned around it to form a cavity, but rather the faces of the active material itself are polished and coated so that they form the mirrors. Monolithic frequency converters are more robust, stable and have less intra-cavity losses than frequency converters with mirrors.
If a cavity-enhanced frequency converter has a single resonance, it means that only one beam is resonant. The remaining beams exit the cavity without making multiple passes though the crystal. If the same source is doubly resonant, it means that two beams are resonant. In case of nondegenerate processes, the source might be triply-resonant. Multiple resonances increase efficiency of the frequency conversion, since the optical intensity inside the cavity is enhanced. A degenerate three wave mixing process means that there are only two (not three) wavelengths involved. Second harmonic generation and degenerate down-conversion in type-I crystals are degenerate processes, so they can be at most doubly resonant. Type I refers to three wave mixing process where the polarizations of all three beams are the same, whereas type II means that one of the frequencies involved is orthogonal to the two remaining ones, rendering the degenerate downconversion process possible only with the type I crystals.
In order to make a second-order-nonlinearity-based device work for a particular set of wavelengths, a phase matching condition must be fulfilled. By phase matching we understand a condition resulting from the principle of conservation of momentum, fulfilment of which is necessary for the three wave mixing to occur. Because of dispersion, different frequency light beams move through the nonlinear medium with different phase velocities, and this creates a momentum (“phase”) mismatch between nonlinearly interacting beams as they pass through the crystal. If not corrected, this phase mismatch leads to a very small net conversion efficiency. A normal technique to ensure the fulfilment of the phase matching condition is periodic poling, by which it is understood that the crystal is fabricated so that the condition is satisfied when the crystal is maintained is a determined phase matching temperature.
Apart from phase-matching, two other kinds of matching conditions must also be satisfied in order to achieve efficient nonlinear conversion in a cavity-based frequency converter. First, resonance conditions, one for each resonated wavelength, must be satisfied, in order to obtain a resonant power build-up. Second, Fabry-Perot cavity based devices with multiple resonances have an additional condition, resulting from the fact that the light passes through the nonlinear medium twice every cavity roundtrip, and the relative phase between the light created in the two consecutive passes through the nonlinear medium must be controlled in order that constructive interference is maintained. By Fabry-Perot cavity we mean a cavity made of two parallel mirrors, for which the light travels twice the same path each roundtrip. This relative phase condition applies only when the pump beam and at least one of the converted beams is resonant in the Fabry-Perot cavity.
For example, in order to obtain a second harmonic generation out of the doubly-resonant linear cavity one needs to control a phase matching condition, two resonance conditions and relative phase conditions which adds up to four degrees of freedom that must be controlled.
There are several methods of tuning the cavities so that the resonance and relative phase conditions are fulfilled, for example                wavelength. Adjusting the laser frequency until it coincides with a cavity resonance frequency. This strategy can be employed when there is no precise requirement of the frequency of the generated light and it can be changed within the cavity free spectral range. Otherwise the cavity resonance must be tuned to the laser frequency using one of the methods mentioned below.        displacing one of the external cavity mirrors by means of an actuator. Moving the mirror so that the cavity length can be controlled.        thermooptical effect. Changing the temperature of an optical element within a cavity changes its refractive index (by different amount for each wavelength involved), modifying the optical path length through the element.        elastooptical effect. Similar to temperature, a pressure applied to an optical element within a cavity also changes the refractive index.        electrooptical effect. The electric field can also change the refractive index.        
To our knowledge, there are no monolithic frequency converters reported in the literature that offer independent control of the phase matching and the cavity resonance(s), as in the invention described here. For example, a monolithic frequency converter (an optical parametric oscillator) described in “Generation of squeezed light with a monolithic optical parametric oscillator (OPO): Simultaneous achievement of phase matching and cavity resonance by temperature control,” Opt. Express 18, 20143-20150 (2010), by Hidehiro Yonezawa, Koyo Nagashima, and Akira Furusawa, requires two conditions to work efficiently, a phase matching and one cavity resonance condition, similarly to a singly resonant second harmonic generation device. In the OPO presented in the Yonezawa et al article, the single resonance condition is satisfied at the cost of the phase matching, since only one degree of freedom is used, namely the temperature of the entire crystal. This presents however the drawback that it is not possible to have independent control of the phase matching temperature and the cavity resonance for a down-converted beam. Therefore, a resonance is achieved at the cost of compromising the phase matching, and thus decreasing the efficiency. Furthermore, in this scenario, adding a resonance of another frequency involved in the nonlinear interaction is impossible.